During semiconductor fabrication, integrated circuits are created on a semiconductor wafer (“wafer”) defined from a material such as silicon. To create the integrated circuits on the wafer, it is necessary to fabricate a large number (e.g., millions) of electronic devices such as resistors, diodes, capacitors, and transistors of various types. Fabrication of the electronic devices involves depositing, removing, and implanting materials at precise locations on the wafer. A process called photolithography is commonly used to facilitate deposition, removal, and implantation of materials at precise locations on the wafer.
In the photolithography process, a photoresist material is first deposited onto the wafer. The photoresist material is then exposed to light filtered by a reticle. The reticle is generally a glass plate that is patterned with exemplary feature geometries that block light from passing through the reticle. After passing through the reticle, the light contacts the surface of the photoresist material. The light changes the chemical composition of the exposed photoresist material. With a positive photoresist material, exposure to the light renders the exposed photoresist material insoluble in a developing solution. Conversely, with a negative photoresist material, exposure to the light renders the exposed photoresist material soluble in the developing solution. After the exposure to the light, the soluble portions of the photoresist material are removed, leaving a patterned photoresist layer.
The wafer is then processed to remove, deposit, or implant materials in the wafer regions not covered by the patterned photoresist layer. Such wafer processing often modifies the photoresist layer in such a way as to make removal of the photoresist more difficult. For example, in the case of a plasma etch process, the outer layer of the photoresist is transformed into a hard crust that is significantly less reactive that the underlying photoresist. After the wafer processing, the patterned photoresist layer, its debris, as well as other types of polymer debris left after plasma etching, need to be removed from the wafer in a process called photoresist stripping. It is important to completely remove the photoresist and polymer material during the photoresist stripping process because such materials remaining on the wafer surface may cause defects in the integrated circuits. Also, the photoresist stripping process should be performed carefully to avoid chemically modifying or physically damaging underlying materials present on the wafer. A need exists for improvement in the photoresist stripping process such that more complete removal of the photoresist and polymer material can be achieved while inflicting less chemical modification and/or damage to the underlying wafer materials.